It was suggested several years ago that psychomotor stimulant drugs may affect neural mechanisms that mediate reward. A recent extension of this hypothesis is that stimulant drugs (e.g., d-amphetamine) may enhance the effects of conditioned reinforcers, stimuli that acquire reinforcing properties by their association with unconditioned reinforcers. Since conditioned reinforcers (e.g., money, grade, informative feedback) play a major role in controlling human behavior it is important to understand mechanisms that might enhance the reinforcing efficacy of these stimuli. Enhancement of conditioned reinforcement might be especially important in attempts to treat individuals with low rates of functional behaviors. The present study will examine the effects of d-amphetamine on responding in multiple, mixed, and second-order schedules, techniques that have been used extensively to study conditioned reinforcement. Two experiments are proposed, both using pigeons as subjects and food as unconditioned reinforcers. A repeated measures, individual-subject design will be used. In Experiment 1 responding in Component 1 of a mixed schedule will produce food or brief stimuli (e.g., a 4-s houselight) according to a second-order schedule. In component 2 responses will never produce food but will only produce the brief stimulus. In some conditions the brief stimulus will be paired with food and in others it will not be paired. d- Amphetamine will be administered in each condition of the experiment and changes in responding for brief stimuli will be examined. d-Amphetamine's effects will also be examined in a multiple schedule where different exteroceptive stimuli signal the food and conditioned-reinforcement components. In Experiment 2 responding will produce brief stimuli and food according to a second-order schedule and the effects of d- amphetamine on rates and patterns of responding will be examined. In some conditions the brief stimulus will be paired with food and in others it will not be paired. Both "salient" and "nonsalient" stimuli will be examined. The experiments should clarify the role that stimulant drugs play in the enhancement of conditioned reinforcement.